Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/28—Manufacture of steel in the converter
  • C21C5/30—Regulating or controlling the blowing
  • C21C5/34—Blowing through the bath

Definitions

• the present invention is concerned with a method of protecting tuyeres for blowing pure oxygen upwardly through the bottom of steel converters.
• the invention is applicable to tuyeres which comprise two or three concentric tubes and which are protected against high-temperature wear by a liquid which contains a hydrocarbon, such for example as fuel oil.
• a known steel converter for refining molten iron into steel has a bottom fitted with a number of tuyeres consisting of two concentric tubes, pure oxygen being supplied to the inner tube and a liquid which contains hydrocarbons being supplied to the space between the two tubes.
• Powdered lime or limestone, or other finely divided materials which are of use in the refining process can be carried in suspension in the oxygen.
• the optimum flow rate for the oxygen at any time depends on the permeability of the oxygen circuit, including that of the tuyeres, on the blowability, i.e. on the amount of splashing produced by the converter, and on the length of time which the blowing is likely to last, taking into account the time required to melt any scrap iron, for example. This is so whether the oxygen is charged with powdered material or not.
• the traditional method in the case of fuel oil consists in maintaining a constant flow of fuel oil throughout the refining process.
• This method has the obvious advantage of simplicity, which accounts for its success. But it also has the disadvantages of being too elementary.
• the flow rate which is generally adopted in using this method which is referred to as the normal flow (NF) rate, is sufficient to effectively prevent the maximal wear of the tuyeres, which only occurs when the carbon content of the molten metal is low, in other words only during the final stage of the process. During most of the blow, before low carbon levels are achieved in the molten metal, this rate of flow is excessive.
• the oxygen flow rate increases, which increases the cooling effect at the tip of the tuyere caused by expansion of the oxygen. This makes the tuyere more resistant to wear, so that there is no need to increase the flow of protective liquid, other things being equal.
• a method of protecting tuyeres for blowing pure oxygen through the bottom of a steel converter comprising supplying a protective liquid which contains hydrocarbons to peripheral passage means in each tuyere and varying the rate of flow of said protective liquid, so that said protective liquid flows at a reduced rate during a first stage, which first stage extends from the beginning of the refining process to a point at which the carbon level in the molten metal is 0.300 to 0.700%, and at a normal rate during a second stage, which second stage extends from the end of said first stage to a point at which 90 to 95% of the total amount of oxygen required for refining the molten metal has been blown in, and at an excess rate during a third and final stage, which third stage extends from the end of said second stage to the point at which all the oxygen has been blown in, said normal flow rate being in the range 0.08 to 0.15 liters per minute per centimeter of the mean circumference of said passage means in
• the normal flow rate is that required for a satisfactory tuyere service life if a constant flow of protective liquid is used throughout the refining process.
• the passage for the protective liquid is a narrow but continuous annulus or a series of discontinuous areas arranged in a ring
• the transverse cross-section of the passage is relatively high, i.e. between 5 and 20 mm 2 per centimeter of its mean circumference, which is equivalent to a gap of 0.5 to 2 mm in the case of two concentric tubes
• the normal flow rate for the protective liquid is between 0.12 and 0.15 liter per minute per centimeter of the mean circumference.
• the normal flow rate for the protective liquid is between 0.08 and 0.12 liter per minute per centimeter of the mean circumference.
• the mean diameter of the annular passage for the liquid is 27.5 mm, and its mean circumference is 86.5 mm, or 8.65 cm.
• the mean diameter of the annular liquid passage is 35 mm and the mean circumference is 110 mm, or 11 cm.
• the method in accordance with the invention may for example be used in the refining of iron containing 1.5 to 2.1% phosphorus (Thomas-Gilchrist process), and in the refining of low phosphorus iron (less than 0.300%). In both cases the end product of the blowing in of pure oxygen is mild steel.
• the molten metal is desilicated and decarburized during said first stage, said first stage extending to the point at which the carbon content of the molten metal is 0.300 to 0.700%, during said second stage decarburization of the molten metal is completed and the greater part of its dephosphorization is carried out, said second stage extending to the point at which 90 to 95% of the total amount of oxygen required for refining the molten metal has been blown in, and during said third stage dephosphorization of the molten metal is completed and the rate of oxidation of the iron is rapidly increased, said third stage extending from the end of said second stage to the point at which said refining process is completed.
• the molten metal is desilicated and decarburized, said first stage extending to the point at which the carbon content of the molten metal is 0.300 to 0.700%, during said second stage the decarburization of the molten metal is completed, said second stage extending to the point at which 90 to 95% of the total amount of oxygen required for refining the molten metal has been blown in, and during said third stage the decarburization of the molten metal is completed and the rate of oxidation of the iron is rapidly increased, said third stage extending from the end of said second stage to the point at which the refining process is completed.
• the third and final stage is omitted, or in other words the protective liquid is not caused to flow at an excess flow rate, the normal flow rate being maintained beyond the point at which 90 to 95% of the oxygen has been blown in, to the point at which the required carbon content of the molten metal is achieved, i.e. until the end of the refining process.
• the flow rate of the protective liquid in each stage is reduced in linear relation to the ratio between the relevant preset value given above and the amount of the powdered material, so that if the amount of powdered material is twice the preset value, the flow rate of the protective liquid is halved.
• the method can be closely adapted to the changing speed at which the tips of the tuyeres are attacked by the iron oxides, in accordance with the decreasing carbon content of the molten metal, and this means that the amount of protective liquid consumed can be significantly reduced in comparison with the conventional method in which the liquid flows at a constant rate throughout the blowing operation.
• the excess flow rate during the third stage has a deoxidizing effect, in addition to its protective effect, during the last few seconds of the blow, and may even have a slight recarburizing effect. This is due to the carbon produced by the cracking of the excess liquid.
• the excess flow does not significantly increase the amount of protective liquid that is used up, as it occurs only during the third and final stage, which is a very short one, accounting for only the final 10% to 5% of the total amount of oxygen blown in.
• the reduction in the flow rate of the protective liquid in response to the amount of powdered lime or limestone in the oxygen passing the preset value contributes to a further reduction in the amount of liquid consumed, while avoiding the formation on the tips of the tuyeres of "mushrooms", or swollen beads of solidified metal, which have an adverse effect on the proper flow of oxygen and on the resistance of the tuyeres.
• Another advantage is that the method is very suitable for automation, i.e. automatic control on the basis of the total amount of oxygen blown since the beginning of the refining process.
• the converter used has a capacity of 60 tons and has seven tuyeres, each of which consists of two concentric tubes, a 28mm/34mm tube for the oxygen and a 36mm/42mm tube for the protective liquid, which consists of domestic fuel oil.
• the normal flow rate NF of the fuel oil is of the order of 1.43 liters per minute in each tuyere, as previously explained.
• the flow rate NF is set at 1.5 1/min/tuyere.
• a phosphorus iron is to be refined into mild (low carbon) steel.
• the iron contains 0.400% silicon, 0.360% manganese, 3.65% carbon, 1.82% phosphorus, and 0.036% sulphur.
• the total volume or oxygen to be blown in, to make 60 tons of steel, is 3360 Nm 3 .
• the three stages are as follows:
• This second stage includes the final phases of the decarburization of the molten metal, and the greater part of its dephosphorization.
• the phosphorus level is not measured at the end of the second stage, but is in the region of 0.170%. In this embodiment, the second stage lasts 3 minutes.
• the phosphorus level of the molten metal is measured at the end of the third stage, and is found to be 0.024%.
• the carbon level is 0.033%.
• the third stage lasts 30 seconds.
• the known method using a constant flow of fuel oil uses up about 2.4 liters per ton of steel in a 60 ton converter.
• the saving provided by use of the above described method is therefore in the region of 1 liter per ton of steel, or some 42%.
• the amount of powdered lime in the oxygen was less than 3 kg/Nm 3 of oxygen, requiring no correction of the fuel oil flow rate. No powdered limestone was used.
• a low phosphorus iron (“haematite” iron) is to be refined, the composition of the iron being: 0.800% silicon, 0.700% manganese, 4.4% carbon, 0.160% phosphorus, and 0.038% sulphur.
• the total volume of oxygen to be blown in, to make 60 tons of steel, is 3 060 Nm 3 .
• the three stages are as follows:
• the carbon level of the molten metal is not measured at this point, but is of the order of 0.600%.
• the first stage lasts nine minutes, and is sub-divided into two parts: during the first part, which lasts three minutes, the amount of powdered lime in the pure oxygen is 4 kg/Nm 3 of oxygen. This is greater than the limit value of 3 kg/Nm 3 , and is required to prevent splashing caused by the high silicon content of the iron. During the final six minutes of the first stage the amount of lime is less than 3 kg/Nm 3 of oxygen.
• the fuel oil flows at the normal rate NF of 1.5 1/min/tuyere, i.e. 10.5 1/min for all seven tuyeres.
• the carbon level of the molten metal is not measured at the end of the second stage, but it is known to be in the region of 0.130%. In this embodiment, the second stage lasts 11/2 minutes.
• the carbon level of the molten metal is measured at the end of this third and final stage, and is found to be 0.034%. In this embodiment, the third stage lasts 30 seconds.
• the amount of powdered lime in the oxygen was less than 3 kg/Nm 3 of oxygen, requiring no other correction of the fuel oil flow rate than that applied during the first three minutes of the first stage.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Carbon Steel Or Casting Steel Manufacturing (AREA)

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Citations (3)

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• 1976
  • 1976-04-28 FR FR7612544A patent/FR2349655A1/fr active Granted
• 1977
  • 1977-03-22 ES ES457064A patent/ES457064A1/es not_active Expired
  • 1977-03-30 DE DE2714131A patent/DE2714131C2/de not_active Expired
  • 1977-04-02 JP JP3801277A patent/JPS52138415A/ja active Granted
  • 1977-04-07 GB GB14914/77A patent/GB1536811A/en not_active Expired
  • 1977-04-12 US US05/786,949 patent/US4081268A/en not_active Expired - Lifetime
  • 1977-04-26 LU LU77205A patent/LU77205A1/xx unknown
  • 1977-04-27 CA CA277,149A patent/CA1082455A/fr not_active Expired
  • 1977-04-28 BE BE177094A patent/BE854057A/xx unknown

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